Photochromic materials are used for producing an optical function for an adaptive light filter or screen, typically an adaptive (UV) sunscreen.
Photochromism is defined as being the property of a material to react to the intensity of a given radiation, typically UV radiation, by modifying its visible-light absorption properties: the material darkens under the effect of the radiation, and lightens in the absence of this radiation. This phenomenon of lightening and darkening is reversible. Photochromic mineral and organic materials are thus found in the glass industry. According to the prior art, mineral or organic photochromic materials are obtained by incorporating photochromic substances into a material, by various known techniques. For example, photochromic organic materials are obtained, before polymerization, by mixing photochromic molecules (for example spirooxazine molecules) with a liquid monomer, or, after polymerization, by surface impregnation via a heat treatment (heat transfer). Photochromic mineral materials will be obtained, for example, by introducing silver halide crystals into the material.
The known photochromic materials have various drawbacks. The manufacturing processes used make them expensive. Notably, optical spectacle glasses incorporating such an optical property are expensive. For these reasons, it is difficult to envisage proposing cheap spectacles, of disposable type, incorporating such functionality, which might, however, be of interest to users of this type of product.
This cost also prevents their use in applications in which the surfaces concerned are relatively large, which limits their potential applications. Now, the integrated optical filter function of these materials is potentially advantageous for many products or articles such as helmet visors, for pilots, for example aircraft pilots, or protective helmets of any type for the construction, public works, sports goggles (cycling), etc. sectors, this list being by no means limiting.
Another drawback of these materials is the relatively slow transition time between the two darkened and lightened states: excitation time, for passing from the resting state to the excited state of the photochromic molecules in the presence of a given radiation, and relaxation time for passing from the excited state to the resting state, in the absence of radiation. These transition times are dependent on the solid nature of the host matrix, mineral or organic glass, which receives the photochromic molecules. Notably, the slowness of relaxation may prove to be a great inconvenience during use. Finally, as regards optical materials, the problem of replacing or repairing them, notably in the event of scratching, still arises.